Semiconductor devices including silicon devices have been developed for high integration and low power consumption by miniaturization of the devices according to a scaling law known as the Moors law that says that “the number of transistors quadruples every three years.” In recent years, the gate lengths of MOS FETs (Metal Oxide Semiconductor Field Effect Transistors) have become 20 nm or less. Because of steep price rises for lithography processes and physical limitations of device dimensions, there has been demand for improved device performance not based on the traditional scale laws.
The steep price rises of lithography processes have resulted from the steep price increase of manufacturing apparatus and mask set. On the other hand, the physical limitations of the device dimensions have resulted from the limitations of operation and dimensional fluctuations.
In recent years, rewritable programmable logic devices called FPGAs, that lie between gate arrays and standard cells, have been developed. FPGAs allow the user to configure circuits after their chips are manufactured. FPGAs are provided with resistance changing elements in their multi-layered wiring layer such that the user himself or herself can configure his or her desired circuit. FPGAs have resistance changing elements in their multi-layered wiring layer such that the user can make electric wire connections in a predetermined manner. Semiconductor devices provided with such an FPGA have improved the freedom degree of circuit design. Resistance changing elements include ReRAM (Resistance RAM (Random Access Memory)) made of a transition metal oxide and NanoBridge (registered trademark of NEC) made of an ion conductor. An ion conductor is a solid state material in which ions can freely migrate by applying an electric field or the like.
As a resistance changing element that is likely to improve the freedom degree of circuit design, a switching element that uses the migration of metal ions and electrochemical reactions in an ion conductor is disclosed in literature (Shunichi KAERIYAMA et al., “A Nonvolatile Programmable Solid-Electrolyte Nanometer Switch,” IEEE Journal of Solid-State Circuits, Vol. 40, No. 1, pp. 168-176, January 2005).
The switching element disclosed in this literature is composed of three layers that are an ion conducting layer, a first electrode, and a second electrode, the first and second electrodes being in contact with two primary surfaces of the ion conducting layer. Among these layers, the first electrode serves to supply metal ions to the ion conducting layer. In contrast, the second electrode does not supply metal ions to the ion conducting layer.
Here, operation of this switching element will be described in brief. When the first electrode is grounded and then a negative voltage is applied to the second electrode, the metal of the first electrode changes to metal ions and dissolve into the ion conducting layer. Thereafter, the metal ions in the ion conducting layer deposit as the metal in the ion conducting layer and the deposited metal forms metal bridges that electrically connect the first electrode and the second electrode. When the metal bridges electrically connect the first electrode and the second electrode, the switching element enters the ON state.
In contrast, while the switching element is in the ON state, when the first electrode is grounded and then a positive voltage is applied to the second electrode, part of the metal bridges is cut. As a result, the electric connection between the first electrode and the second electrode is cut and thereby the switching element enters the OFF state. Before the electric connection is completely cut, electric characteristics change, for example, the resistance between the first electrode and the second electrode becomes large and the capacitance therebetween changes and finally, the electric connection is cut. To change the state of the switching element from the OFF state to the ON state, the first electrode is grounded and then a negative voltage is applied to the second electrode.
In addition, the foregoing literature discloses a structure and operation of a two-terminal type switching element that has two electrodes faced each other through an ion conductor and that is operated by controlling the conducting state therebetween.
Such a switching element is smaller and has a lower on-resistance than a semiconductor switch such as an MOS FET. Thus, it is thought that such a switching element has a potential to be applied to programmable logic devices. In addition, since this switching element can maintain the conducting state (ON or OFF state of the element) without the need to apply a voltage, it may be used for a nonvolatile memory element.
For example, a plurality of memory cells as basic elements each containing one selector element and one switching element composed of for example transistors are arranged in the vertical and horizontal directions. When the memory cells are arranged in such a manner, any one of a plurality of memory cells can be selected using word wires and bit wires. As a result, a nonvolatile memory that senses the conducting state of a switching element of the selected memory cell and reads information of “1” or “0” based on the ON or OFF state of the switching element can be realized.